Numerous devices have been proposed which provide a visual indication of the passage of a pre-arranged amount of time.
Time indicators are useful when attached to perishable items in indicating the length of time the items have been on the wholesaler's or retailer's shelf. Thus, foods and other perishable items such as photographic materials can be provided with indicators which evidence a visual change, after being activated, after the passage of a predetermined period of time.
Time-temperature indicators are also known which may indicate a visual change as a function of both time and temperature. Such devices are useful when attached to frozen foods since it may be undesirable to subject such frozen items to both high temperatures for a short period of time and relatively low but above-freezing temperatures for relatively longer periods of time. Further, the frozen food may be storable indefinitely at a very low temperature and the time-temperature indicator should take this all into account.
It is apparent that even straight time indicators which are not described in terms of giving a visual change dependent upon temperature are nevertheless somewhat dependent in yielding a visual change upon the temperature. Thus, while time-temperature indicators may utilize the freezing and thawing phenomenon of a particular material to incorporate the temperature factor into the scheme of when the indicator should produce a visual change, virtually any mechanism which is indicated as being dependent upon time is also, at least to a minor extent, dependent upon temperature.
A straight time indicator is described in U.S. Pat. No. 3,962,920 wherein a reservoir pad of paper or other coarse absorbent material is separated from a wick, such as a piece of blotter paper having fine porosity, by an impervious film to prevent contact between the reservoir pad and the wick. After the impervious films is removed, the agent in the reservoir is allowed to travel through the wick whereupon the progress of this movement is observed through a clear window. The agent can be a dye or one member of a co-reactant pair, the other member of which is in the wick. Alternatively, the device can be prepared without an impervious film at a temperature where the agent is a solid whereupon the device would be considered more of a time-temperature indicator.
Other wicking devices are described in U.S. Pat. Nos. 2,896,568, 3,046,786, 3,243,303 and 3,954,011.
Frozen food tell-tale devices are described in U.S. Pat. Nos. 2,951,764, 3,118,774 and 3,414,415. Such devices indicate that the food package to which it is attached has been subjected to a thawing temperature or a thawing temperature for a period of time sufficient to indicate disposal of the item.
An acid-base type indicating mechanism for a frozen food tell-tale device is described in U.S. Pat. No. 2,716,065. A more recent application of the acid-base indicating mechanism is set forth in U.S. Pat. No. 3,942,467 which also contains a review of this technology.
The use of a gaseous agent to produce a color change is described in U.S. Pat. No. 3,996,007. The gas is generated in a rate-controlling pouch and is either acidic or basic. After passing through the rate-controlling film, the gas comes into contact with an absorbent wick containing a pH sensitive dye and the other component of the acid-base pair.
A straight time indicator is described in U.S. Pat. No. 4,028,876 wherein one agent is contained in a capsule immersed in a liquid in a transparent pouch. After breaking the capsule, the agent is diluted in the liquid and permeates another capsule to change the liquid color.
Other time indicators are described in U.S. Pat. Nos. 1,843,234, 3,479,877, 2,553,369 and 2,560,537.
However, a need exists in the art for a time indicator which is structurally integrated, solid but with any desired degree of flexibility and which is resistant to breakage or partial breakage whereby the indicating mechanism would be prematurely initiated.